In wavelength division multiplex transmission systems, it is essential to reliably obtain laser lights with a number of close wavelengths. For transmission tests or tests of optical components used in wavelength division/multiplex transmission systems, there is the need for a laser light source highly stable in wavelengths and outputs.
ITU has recommended 0.8 nm (100 GHz) as the wavelength interval in wavelength division multiplex transmission systems. While temperature coefficients of wavelength changes of semiconductor lasers are approximately 0.1 nm/.degree. C. That is, semiconductor lasers are very sensitive to temperature fluctuation. Therefore, it is difficult to maintain wavelength intervals of 0.8 nm in a number of semiconductor laser light sources over a long period. Moreover, in ordinary laser sources, injected current is used to stabilize optical outputs. Control current for stabilization of optical outputs causes changes in temperature, and it results in changes in wavelength. That is, control of optical outputs affects wavelengths, and makes it difficult to stabilize wavelengths.
A prior proposal to cope with the problem is to connect an optical filter and an optically amplifying element in a ring to form a multi-wavelength light source for collectively supplying multiple wavelengths. FIG. 15 is a schematic block diagram showing a prior example A Fabry-Perot optical filter 210, erbium-doped optical fiber amplifier 212 and optical fiber coupler 214 are connected to form a ring.
FIG. 16 show characteristic diagrams of the prior example of FIG. 15. FIG. 16(1) shows transparent wavelength characteristics of the Fabry-Perot optical filter 210, FIG. 16(2) shows amplifying characteristics of the optical fiber amplifier 212, and FIG. 16(3) shows the spectral waveform of output wavelength. The Fabry-Perot optical filter 210 is a kind of wavelength selecting optical filters having wavelength transparent characteristics which permit specific wavelengths in certain wavelength intervals called FSR (Free Spectral Range) to pass through as shown in FIG. 16(1). Individual transparent wavelengths of the Fabry-Perot optical filter 210 are selected from the spontaneous emission light generated in the optical fiber amplifier 212. The output spectral waveform coincides with that obtained by multiplying the transparent wavelength characteristics of the optical filter 210 by the amplifying characteristics of the optical fiber amplifier 212. Theoretically, laser oscillation outputs are obtained in wavelengths where the gain of the optical fiber amplifier 212 surpasses the loss of the optical loop.
In the prior art example shown in FIG. 15, the output intensity is large near the gain center wavelength within the amplifying range of the optical fiber amplifier 212, where oscillation is most liable to occur, and largely decreases in peripheral portions, as shown in FIG. 16(3). That is, the prior art example cannot realize simultaneous oscillation in multiple wavelengths in substantially uniform output levels.
Moreover, wavelength interval in output light in the prior art example exclusively depends on transparent characteristics of the Fabry-Perot optical filter 210. When the wavelength interval is 0.8 nm (100 GHz), the wavelength interval FSR of the transparent wavelength characteristics of the Fabry-Perot optical filter 210 is less than the uniform extension width of the erbium-doped optical fiber amplifier 212. Therefore, even when a plurality of oscillation wavelengths are obtained near the gain center wavelength of the erbium-doped optical fiber amplifier 212, mode competition occurs, and results in unstable output intensities and wavelength fluctuations of respective wavelengths.
A Fabry-Perot semiconductor lasers is a multi-wavelength light source, other than the fiber ring light source. However, it involves unacceptable fluctuations in oscillation wavelengths due to mode competition or mode hopping, and fails to uniform intensities of respective oscillated wavelength components.